The declaration of a major incident activates contingency protocols that compel staff to turn up for work.

But union Unite accused the Ambulance Service of "cynically exploiting" the major incident protocols.

Unite responded to the declaration by advising its members to return to work.

The service said on Thursday night it had been forced to declare a major incident to maintain a safe level of ambulance cover throughout the planned period of industrial action on Friday.

An NIAS spokesman said: "The Northern Ireland Ambulance Service has tonight been forced to declare a major incident to maintain a safe level of ambulance cover throughout the planned period of industrial action on Friday 13 March.

"An already precarious position following agreement with trade unions has deteriorated to a situation where levels of cover are critical from midnight tonight.

"Belfast, Derry, Omagh Enniskillen, Lisburn are among towns that would be without ambulance cover from midnight with other areas being left with seriously reduced levels of cover. This has placed these communities at serious risk in terms of ambulance provision.

"Despite their trade unions agreeing that staff would respond to life threatening emergencies the Trust has been inundated with staff phoning in to say 'I am withdrawing my labour'.

"The Trust has been unable to illicit timely responses from trade union colleagues to communications informing them of the situation.

"NIAS has exhausted all alternative contingency options and lives would be at risk if we allowed the situation to deteriorate further. The Trust would expect staff to report for duty in response to this major incident in line with protocols agreed with trade union representatives."

But Kevin McAdam, Unite national officer with responsibility for health, said the union was "unhappy and disgusted" at the Trust's response.

"We advised our members to return to normal roles in response to the NIAS's action in line with undertakings we had given," he said.

"The trade unions know now the facts on the ground: that there was no major incident, rather there was an inability and unwillingness for management to effectively manage the strike.

"We are unhappy and disgusted by the NIAS cynically exploiting this part of the emergency cover understanding to thwart the industrial action.

"We had no choice but to respond as we did as our representatives were unable to get genuine emergency cover. After management have pulled a stunt like this, such cover could not have been guaranteed and could have led to the trust putting the public's lives at risk.

"This is a demonstration yet again of the operational management failure to address the needs of the service when faced with difficulties.

"Our members spent two days working with managers to ensure emergency provision and indeed were working to ensure enough crews were available at sites when management unilaterally declared a major incident.

"The loser in this situation will be future industrial relations in the ambulance service. From here on we reserve the legal right to strike and NIAS have a statutory obligation to provide a service, nothing further needs accommodated."